In general, the processing of black and white silver halide photographic materials is carried out in the order of development, stopping, fixing and washing.
The development of said photographic materials is typically carried out with aqueous alkaline developing solutions comprising one or more developing agents. These solutions are conventionally relatively dilute, therefore it is not generally feasible, from an economic standpoint, to package, transport and store developing solutions of working strength since this would involve the packaging, transporting and storage of large amounts of water. Heretofore, there have been four distinctly different approaches taken to the problem of packaging photographic developing compositions in a form that is suitable for transporting and storage. The first approach involves the preparation of dry powder mixtures which must be dissolved in water and then diluted to the proper volume before use. The second approach involves the formulation of photographic developing compositions in tablet form. The third approach involves the preparation of photographic developing concentrates as paste-like compositions. The fourth approach involves the preparation of liquid concentrates, that is concentrated solutions which merely have to be diluted with water to obtain a working strength (ready to use) solution.
Examples of such packaged developing formulations are described in numerous patents and publications, such as the dry powder developer mixtures in U.S. Pat. Nos. 2,843,484 and 2,846,308, the tablet form in CA patent No. 831,928, the paste-like compositions in U.S. Pat. Nos. 2,735,774, 2,784,086 and in IT patent No. 427,967, the developer concentrates in U.S. Pat. No. 3,178,284, 3,467,521, 3,532,498, 3,589,902, 3,854,948 and 4,046,571, in GB patent No. 1,372,755 and in CA patent No. 998,551.
Each of the prior art approaches possesses both advantages and disadvantages. For example, the use of dry powder mixtures avoids the expense involved in shipping and storing of water and enables the preparation of compact light-weight packages that require little storage space. However, dry powder mixtures are highly disadvantageous in that they are difficult to handle, create a hazard as a result of the generation of dust, require highly accurate weighing and dispensing techniques which are difficult to control, and often require difficult and time-consuming procedures as to bring about dissolution in water during preparation of the working strength solution.
On the other hand, the formulation of photographic developing compositions in tablet form has many advantages but is not, in general, a feasible commercial alternative to the use or other techniques, because the preparation of tablets is a very complex and expensive procedure, and many photographic developing compositions are not suitable to tablet formation. Moreover, tablets usually require the use of large amounts of binding agents, and these binding agents can make the dissolution of the tablets difficult and/or cause adverse sensitometric effects in processing. One approach to the problem of promoting prompt dissolution of the photographic processing tablets is to incorporate effervescing agents. However, the use of these materials is generally not very effective and introduces additional costs and complexity in the manufacturing operation.
The use of developing concentrates of paste-like consistency suffers from such problems as poor flow characteristics which render them very difficult to handle, lack of chemical stability, a tendency to separate, cake or crystallize, and poor solubility characteristics, so that they have been difficult to dissolve. The use of suspending, binding, thickening or gelling agents adds substantially to the cost and complexity of the manufacturing operation and can create problems of microbilogical growth formation and the formation of scums and residues in processing operations. Moreover, there are very few, if any, such agents that will function effectively under conditions of high pH and/or high salt concentration. In some instances, paste-like developing concentrates have been prepared without the use of suspending, binding, thickening or gelling agents, but these compositions have exhibited similar disadvantageous dissolving characteristicas.
At last, liquid concentrates are very convenient to use and much less hazardous, since they eliminate the dust problem, they can be very readily diluted to working strength solution without the need for laborious mixing operations and they do not need particular additive agents. In spite of this, there is a severe problem involved with such liquid concentrates of photographic developing compositions as there is a tendency for chemical interactions or modifications to take place between certain of the components in a single liquid concentrate. This frequently prevents combining all of the components in a single liquid concentrate, and necessitates the separation of the components into two or more parts which must be subsequently combined to form the final processing solution. This greatly complicates the packaging operation and adds to the expense of manufacture, transport and storage. A solution for this problem is disclosed in U.S. Pat. No. 3,178,284 or in EP patent application Ser. No. 136,582, but a limit for the maximum concentration is involved with the solubility of the components within the condition of storage and use. In particular this problem involves the quantity of developing agents (such as dihydroxybenzene developing agents, e.g. hydroquinone) and of inorganic antioxidant agents (such as alkali metal sulfites and metabisulfites) as disclosed in U.S. Pat. No. 3,589,902 column 1 lines 45-64, U.S. Pat. No. 3,467,521 column 1 lines 25-31, GB patent No. 1,372,755 page 1 lines 9-14 and CA patent No. 998,551 page 1 lines 9-16, here incorporated for reference. Additionally, in the use of alkali metal sulfites and metabisulfites, the cheaper sodium salts are poorly soluble in the highly concentrated alkaline developer solutions (less than 7% by weight), so that it is necessary to use the more soluble (about 25% by weight) but expensive potassium salts.